Frequency modulation at ultra high frequencies



. Sept. 28, 1948.. 2,450,182

FREQUENCY MODULATION AT ULTRA HIGH FREQUENCIES Filed March 26, 1943 F.L. BURROUGHS 2 Sheets-Sheet 1 Cav s w m N a mM/m 3% w 5 %A A. m m

' F. 1.. BURROUGHS 2,450,182

2 SheeisSheet 2 LTL Sept. 28, 1948.

FREQUENCY MODULATION AT ULTRA HIGH FREQUENCIES Filed March 26, 1943FIIIIITIIIIIIIL I Patented Sept. 28, 1948 FREQUENCY MODULATION AT ULTRAHIGH FREQUENCIES Franklin L. Burroughs, Emporium, Pa., assignor toSylvania Electric Products Inc., Emporium, Ta, 2. ccrporationofMassachusetts Application March'26, 1943,, SerialNo. 480,611

4 Claims.

This invention relates to frequency modulation, particularly to that ofoscillation circuits generating frequencies of the order of severalhundred megacycles.

The attempts at applying frequency modulae tions to carrier frequenciesof several hundred mega-cycles have not been successful. Conven tionalreactance tube modulating circuits cannot be used, partly because of theerratic behavior of lumped impedances at these frequencies, partly as aresult of the unavoidable straycapacities and inductances .of thecircuit leads, and finally due to the fact that anode and grid voltagesat these frequencies are no longer in phase opposition. This latterchange of the phase relations between input and output voltages of atube is attributable to transit time effects, and it may be said thatthe increase of the electron transit time with frequency has formed abarrier for the application of frequency modulation to the higherfrequencies.

The invention is based on the principle of usefully employing these sameelectron transit time effects for obtaining frequency modulation atthose high frequencies for which up to now such effects have formed abarrier.

It is, accordingly, a principal object of the invention to provide meansfor utilizing electron transit time effects for the purpose offrequencymodulating ultra-high frequency carriers.

One object of the invention relates to the translation of transit timeeffects into variation of a shunt capacity applied to a tunedtransmission line section.

Another object of the invention relates to the modulation of the inputcapacity of a modulator tube, part of which is effectively in parallelwith a fixed shunt capacity applied to a tuned transmission linesection.

It is an object of the invention to provide means for applying an ultrahigh frequency'voltage to the input electrode of an electron dischargetube and to change the transit time of the electrons between these inputelectrodes by means of a low frequency voltage, whereby the frequency ofthe ultra high frequency voltage may be changed within wide limits in anassociated tuned transmission line circuit.

It is another object to provide an ultra high frequency oscillationcircuit whose frequency is varied in response to low frequency voltagesapplied to the input electrodes of an auxiliary discharge tube.

.It is :a still further object of the invention to utilize the variationof the input capacity of :an

- changing the electron discharge tube, caused by variation of thetransit angle of the electrons traveling from one input electrode ofthis tube to another, for the purpose of modulating the frequency of atuned transmission line section.

One feature of the invention relates to the de tuning of an oscillationgenerator, consisting of a tuned quarter-wave transmission line shuntedby the plate to grid capacity of an oscillator tube, which detuning iscarried out by properly coupling the variable input capacity of anauxiliary modulator tube to the electrodes of the oscillator tube.

A still further feature relates to modulating the frequency of a tunedtransmission line sectionfby transit angle of the electrons between twoelectrodes in one or more of the tubes which are coupled to thetransmission line.

The lowest natural frequency 710 of a transmission line section dependson its length s and theboundary conditions. For instance, a quarterwavesection is formed by a transmission line of length s, closed on one endand open on the other. Similarly, a half-wave section is formed by atransmission line of length s whose ends are either both open or bothshort-circuited. The frequency no is connected with the wave lengths heby the well known relation no=clo, where c is the velocity of light.Another example of a transmission line section whose frequency caneasily be determined is the transmission line of length 8, closed on oneend and shunted at the other by a capacity KO. This type of transmissionline section is usually called a quarter wave by a capacity. A simpleformula can be written for the natural frequency of this type oftransmission line if the unit length capacity K of the trans-missionline is known. The natural frequency of such a shunted line section isthen given by the expression (1) a tan a: KS/Ko where w is equal to21rnos/ 0. Equation 1 shows that a variation of the shunt capacity K0will vary the frequency of the transmission line section.

If K0 consists of the interelectrode capacity between the plate and gridof an oscillator tube and the input electrodes of a modulator tube whichis coupled to the former by coupling condensers, the total shuntcapacity K0 can be varied by varying the input capacity of the modulatortube.

According to the invention, the input capacity of the modulator tube isvaried by superimposing a low frequency modulating voltage on the ultrahigh frequency voltage applied to the modulator line shunted grid. Theinput capacity of the modulator tube varies as a function of the transittime of the electrons traveling from cathode to grid and of the ultrahigh frequency voltage applied to the grid.

The product of the ultra high circular frequency o and the transit timeT is usually called the transit angle If the applied ultra highfrequency voltage amplitude is small compared with the effective D. C.voltage between cathode and grid, the effective input capacity Cv can beexpressed in the form where Cov is the cold cathode to grid capacity (ofa triode) M is usually of the order of A; and the phase angle {3 is O,for =O, and increases slowly with increasing transit angle up to alimiting value of about 31r/2. (The exact values of M and ,8 depend onthe tube construction and the operating conditions, and they are notsignificant for an understanding of the invention. For limitations andthe derivation of Equation 2, see, c. g., the article by H. Rothe, P. I.R. E., July, 1940, pp. 325 if.)

The invention will now be described in connection with the drawing, inwhich,

Fig. 1 is a curve of the input capacity as a function of the transitangle.

Fig. 2 is a simplified equivalent circuit diagram of the circuitelements shown in Figs. 3 and 5.

Fig. 3 represents a circuit according to one embodiment of theinvention.

Fig. 4 is an exact equivalent capacity diagram corresponding to theembodiment shown in Fig. 3.

Fig. 5 represents another embodiment of the invention.

Fig. 6 is an exact equivalent capacity diagram corresponding totheembodiment shown in Fig. 5.

The curve of Fig. 1 shows the general character of the variation of theinput capacity cv of an electron discharge tube (e. g., a triode) as afunction of the transit angle Let the transit time T of the electronsfrom cathode to grid be decreased,- which may be achieved by making theeffective D. C. voltage more positive. If the ultra high frequencysuperimposed on this D. C. voltage remains constant, equal to we, thevariation of the transit angle corresponding to a variation of transittime dT will be do=wodT. In the circuits shown in the drawing, avariation of the transit time changes the circular frequency from we towo-l-dwo because, according to the invention, part of the input capacityof the modulator tube is in parallel with the shunt capacity of thetransmission line. Thus the total change of transit angle becomes Byinspection of Fig. 1, it can be seen that for moderate values of 11 thevalue of the slope of cv, i. e.,

In Fig. 3 are two tubes, tube A with cathode F1, grid 01, and plate P1,and tube B with corresponding cathode F2, grid 92, and plate P2, withthe grid 92 of B coupled to the plate P1 of tube A. Thecapacity betweenF2 and gm of tube B is variable capacity Cv, while that between P1 andyr of tube A is constant capacity C1. In Fig. 3, the microphone M isconnected through a suitable coupling unit T, which may include apreamplifier, to the transmission line L shunted by a condenser K. Theline L may in conjunction with condenser K, be adjusted to act as aquarterwave line and also to serve as an ultra high frequency trapbetween the device 'I' and tube B while allowing the signals from deviceT to be impressed upon the said tube. If desired, each of the linesections L may be an adjustable coaxial line. One side of thequarter-Wave line is connected to grid g2, and the other side of theline is connected to cathode F2 through a suitable grid bias battery Bor the like. Preferably, battery B is so poled as to normally bias thegrid 92 negatively with respect to the cathode F2. As the signal voltagefrom device T are impressed on grid 92, the transit angle between 92 andF2 is correspondingly varied as above described, thus varying the inputcapacitance between 92 and F2. The plate or anode electrode P2 isconnected to a suitable source of steady plate voltage +B through thehigh frequency choke CH. The inherent electrostatic capacitance betweenthe plate and grid is represented as Bpg, and the inherent electrostaticcapacitance between the plate and cathode is represented by Bpf. Thevariable input capacitance between 92 and F2 is represented by Cv. Thegrid 92 is coupled through condenser C01 to the plate or anode electrodeP1 of tube A which is likewise supplied with steady plate voltagethrough the high frequency choke coil CH1. The grid g1 in returned toground through the grid leak resistor GL in series with the choke coilCH2. Tube A is connected to act as a high frequency oscillator by reasonof the couplling between the plate P1 and 91 by means of the tunedtransmission loop S, one side of which is connected to the :plate P1through the block'condenser BC. The loo S may be of an adjustable lengthtype forming in effect an oscillator loop having the desiredself-inductance and capacitance for controlling the generation of theoscillations at the desired frequency. A coupling loop Lp is locatedadjacent to line loop S and the loop 11p is connected to a suitabledi-pole antenna D Fig. 5 is a modification of Fig. 3 and the partsthereof corresponding to those of Fig. 3, bear the same designationnumerals. The main difierence between the embodiment of Fig. 5 and thatof Fig. 3 is that the grid of tube B is coupled through the condenserC02 to the grid Of tube A instead of to the plate of the latter tube.

Fig. 2 is a simplified diagram of the capacity coupling corresponding tothe two embodiments of the invention indicated in Figs. 3 to 6. As caneasily be found by a comparison of Figs. 2 to 4, the capacity Cl. ofFig. 2 is identical with capacity C1 of Fig. 4. Capacity C'v of Fig. 2isthe eifective capacity of Cv in Fig. 4, in parallel with the twoseries connected capacities Bpg and Bpf. Ch of Fig. 2 corresponds to Cof Fig. 3

and Ck2' of Fig. 2 is equivalent to Ca of Fig. 3. The capacity of Cpl ofFig. 3 i assumed to be negligible in this comparison.

It is thus evident that the embodiments shown in Figs. 3 to 6 may berepresented by the simple diagram shown in Fig. 2. In Figs. 3 to 6, thecapacity C01 and CO2 are external coupling capacities coupling one ofthe electrodes of the oscillator tube A to the grid of the modulatortube B. Bpg and Bpf are the plate to grid and the plate to cathodecapacities of tube B respectively. In these figures, the line S may, forexample, be a Lecher Wire system, coupling in loop LP to antenna DP.

According to the invention, grid 92 is preferably at a fixed bias andmodulated by low frequency voltages which may be fed from the microphoneM or any other low frequency source. The coupling capacities C01 and Cc2are properly chosen, and tube types must be selected which will yield areasonable variation of the effective capacity in response to variationsof the capacity Cv.

The embodiments shown in the figure are given for the purpose ofillustration only and not by Way of limitation. Modifications whichoffer themselves to those skilled in the art are considered to be Withinthe scope of the invention. In place of a circuit using a singlemodulator tube, another circuit may be substituted by the propercombination of two push-pull modulators, and the coupling capacities maybe applied at appropriate points of the tuned transmission line ratherthan at the ends as shown in Figs. 3 and 5. The proper choice of thetube types incorporating the characteristics concerning the influence ofthe efiective coupling capacity on the deviation ratio, the applicationof the methods for preventing frequency drift and similar changes,desirable from a practical point of view in a given application, may becarried out without deviating from the spirit of the invention. In manycases, it will be satisfactory to make tube B an ordinary pentode ortriode whose cathode to grid capacity remains positive duringmodulation. For larger deviations a tube may be used with a space chargegrid located between the cathode and control grid and the operatingvoltages are chosen so that the effective input capacitance variesbetween positive and negative values.

What I claim is:

1. In combination, an oscillator tube, a modulator tube, and a tunedtransmission loop having connected thereto a frequency-determiningcapacity network comprising four capacitances, one

of the capacitance being in shunt to the loop the other threecapacitances being in series with each other, the series capacitancesalso being in shunt to said loop, said one capacitance comprising theinterelectrode inherent capacitance between two of the electrodes of theoscillator tube, said other three capacitances comprising in seriesrespectively the grid-cathode capacitance of the oscillator tube, thegrid-cathode capacitance of the modulator tube and a separate condenser,and means to vary the bias on the grid of the modulator tube to varythereby the effective capacitance of said network and thereby varyingthe frequency of the oscillations in said loop.

2. The combination according to claim 1 in which the said condenser isconnected between the plate of the oscillator tube and the grid of themodulator tube.

3. The combination according to claim 1 in which the said condenser isconnected between the grid of the oscillator tube and the grid of themodulator tube.

4. The combination according to claim 1 in which a tuned quarter-wavetransmission line is included between the grid-biassing means and themodulator tube and acts as a Wave trap for the high frequencyoscillations.

FRANKLIN L. BURROUGHS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,777,410 Jones (Dot. 7, 19301,945,545 Finch et al. Feb. 6, 1934 1,945,547 Pray Feb. 6, 19341,950,759 Terman Mar. 13, 1934 2,032,620 Langmuir Mar. 3, 1936 2,231,372Rothe et a1 Feb. 11, 1941 2,278,429 Crosby Apr. 7, 1942 2,305,882Lindenblad Dec. 22, 1942 OTHER REFERENCES Ultra-High FrequencyTechniques, by Brainerd, et al., published by Van Nostrand (30., NewYork city, pages 82, 83, 296, and 297. Copy in Division 51.

